The use of semipermeable membranes for reverse osmosis or ultrafiltration processes is well known. For example, in a reverse osmosis process saline water may be passed through a semipermeable membrane which is permeable to water but relatively impermeable to salt, thereby separating the brine in the water from the water to afford relatively pure water which may be utilized for personal use such as drinking, cooking, etc.
In addition to the separation of liquids, certain membranes may also be utilized for the separation of various gases. The separation of a gas mixture utilizing a membrane is effected by passing a feed stream of the gas across the surface of the membrane. Inasmuch as the feed stream is at an elevated pressure relative to the effluent stream, a more permeable component of the mixture will pass through the membrane at a more rapid rate than will a less permeable component. Therefore, the permeate stream which passes through the membrane is enriched in the more permeable component while, conversely, the residue stream is enriched in the less permeable component of the feed.
This ability to separate gases from a mixture stream will find many applications in commercial uses. For example, gas separation systems may be used for oxygen enrichment of air, for improved combustion efficiencies and conservation of energy resources. Likewise, nitrogen enrichment of air may be applicable where inert atmospheres are required. Other applications for oxygen enriched gases may be improving selectivity and efficiency of chemical and metallurgical processes. Similarly, inert atmospheres such as may be provided for by this invention may also be utilized in chemical and metallurgical processes. Some other applications of gas separation would include helium recovery from natural gas, hydrogen enrichment in industrial process applications, and scrubbing of acid gases. Specific uses for oxygen enrichment of air would be breathing systems for submarines and other underwater stations, improved heart-lung machines, and other lung assist devices. Another specific application of a gas separation system would be in aircraft to provide oxygen enrichment for life-support systems and nitrogen enrichment for providing an inert atmosphere for fuel systems. In addition, gas separation systems may be used for environmental benefits, e.g. methane can be separated from carbon dioxide in waste gases from sewage treatment processes and oxygen enriched air can be produced to enhance sewage digestion.
Among the thin film polymers which may be used for gas separation membranes are the poly(methylpentenes). However, the methylpentene and particularly 4-methyl-1-pentene which is used to prepare the desired membrane must possess certain characteristics which will enable it to be utilized in the desired manner. Some prior methods to obtain a poly(methylpentene) utilized various catalysts to effect the polymerization. One prior method employs a Ziegler-Natta type catalyst. When utilizing a compound such as aluminum chloride in the polymerization reaction, the poly(methylpentene) is in the form of a sticky solid which is unsuitable for use in the preparation of membranes. Likewise, a catalyst comprising aluminum triisobutyl-titanium tetrachloride produces a low molecular weight solid but brittle polymer which is also unsuitable in the preparation of membranes. In a similar manner, commercial poly(methylpentene) when polymerized from 4-methyl-1-pentene uses a titanium chloride catalyst which has been activated with aluminum trialkyls, producing a polymer which is largely an isotactic material which possesses a poor solubility in solvents as well as being brittle and relatively opaque in nature.
In view of the disadvantages of the prior methods, it is necessary to effect the polymerization of 4-methyl-1-pentene under certain conditions whereby a polymer is obtained which may be utilized to prepare a membrane which will effectively act to separate various gases. In this respect, it has been discovered that undesirable characteristics present in the polymer which is obtained by the polymerization reaction may be traced to impurities which are present in the solvent which is employed as a medium in which the polymerization is effected. In order to overcome these disadvantages, it is necessary to utilize a solvent which is free from impurities which led to the preparation of unusable polymers.
As will hereinafter be shown in greater detail, it is now possible to purify a solvent such as cyclohexane whereby it may be effectively used in polymerization reactions involving olefins such as 4-methyl-1-pentene.